Communication systems link together two communication devices so that the devices can send information to each other in a call or other communication event. Information may include voice, text, images or video.
One such communication system is a peer to peer communication system, in which a plurality of end users can be connected for communication purposes via a communications structure such as the internet. The communications structure is substantially decentralised with regard to communication route switching therein for connecting the end users. That is, the end users can establish their own communication routes through the structure based on exchange of one or more authorisation certificates (user identity certificates—UIC) to acquire access to the structure. The structure includes an administration arrangement issuing the certificates to the end users. Such a communication system is described in WO 2005/009019.
Peer-to-peer systems and other communication systems that use the internet or any other packet switched network employ voice over IP (internet protocol) protocols (VoIP) to transmit data. These systems are commonly referred to as VoIP systems. VoIP systems are beneficial to the user as they are often of significantly lower cost than communication networks, such as fixed line or mobile networks, otherwise referred to as public switched telephone networks (PSTN). This may particularly be the case for long distance calls.
The connection of two terminals using more than one type of network is becoming increasingly common. For example communication may be routed via a private network and a public network such as the internet. This may occur when each terminal is located in different networks, or in an attempt to reduce the charges for routing the communication.
A private network, such as a private local area network (LAN) may be connected to the internet. In order for a device located in the private network to communicate with a node located in the internet, a gateway is used to provide the private network with a public address that is identifiable in the internet. Typically this gateway will be a network address translation (NAT) device or a proxy node.
In some cases networks may be connected by gateways that restrict the communication between the networks such that the gateway will only allow a connection between the networks to be established in one direction. Such gateways will hereinafter be referred to as restrictive gateways.
Restrictive gateways may allow devices in a private network to control when they receive data from another network. If a node in a public network is connected to a device in a private network via a restrictive gateway which prevents a communication being initiated by the node to the device, it will be necessary for the node to wait until the device in the private network establishes a communication with the node before the node can transmit data to the device. For example, if a node in the public internet requires information from a device located in a private network, then the node has to wait until the device establishes a connection with the node.
Restricting the direction in which a communication may be established is not limited to cases where the communication is established across two networks.
For example an operator of a network may choose to limit the direction in which a connection is established from a network node. For example, outgoing communications from the node may be prevented in order to avoid incurring the cost of the connection. In this case a connection may only be established with another party if the connection is established by the other party.
Restricting the direction in which a connection may be established from a node can present a number of problems, particularly in the case where the node is intended to route data to other devices. For example, data may need to be queued in a node of the transmitting network thereby stretching the resources of the transmitting node. This can be extremely problematic if the node is not able to determine when the receiving node will initiate a communication as the transmitting node will be unable to manage its resources efficiently.
A further problem presented by a restrictive gateway is that the transmission of data from the transmitting node may be required by the receiving node before the receiving node initiates the communication. For example the transmission of data from the transmitting node may be timed out before the receiving node initiates communication with the transmitting node.
One method for controlling the amount of data which is transmitted across a restrictive gateway is to require the devices in the private network to periodically establish contact or ‘poll’ the node located in the public network. However this solution places a burden on the resources of the devices required to poll the node.